Section 19.3.3.3.3. Designing experiments

The angular range for data collection should first be determined on the basis of the structural information required, and an instrument should be configured accordingly. Most camera systems have the flexibility to use more than one sample-to-detector distance, and they are occasionally equipped with an additional detector to cover higher angles.

The exposure time is usually determined empirically, taking into account the data statistics required, the number of conditions to be investigated and the total amount of beam time available. Typical exposure times run from a few minutes with a synchrotron source to several hours with a laboratory source. Exposure time depends strongly on the sample molecular weight, the concentration and the angular scattering range of interest. It is essential to measure the blank solvent, i.e., the buffer solution in which the protein of interest is dissolved. It should be recorded with the same statistical significance as the protein solution scattering. This blank is subtracted from the observed protein solution scattering intensity, and therefore its intensity contributes to the overall counting statistics. Accurate blank measurements can take significant amounts of time and should not be ignored when planning the experiment. Blank solution scattering patterns should be recorded as often as possible. They serve as an internal control to detect systematic errors during periods of instrumental instability. In addition, the intensity of the incident X-ray beam should be integrated during the exposure time and used to scale the scattering data. Although it is useful to record the beam intensity transmitted through the solution sample, absorption is generally not a significant problem for solution scattering.